1. Field of the Invention
The present invention relates to a vehicular suspension system employing a plurality of variable damping force shock absorbers, each including a variable semi-active hydraulic damper which is optimally applied for suppressing bouncing, rolling, and pitching motions of the vehicle, and specifically to a system which is capable of actively and smoothly controlling damping coefficients of plural semiactive hydraulic dampers depending upon a vertical velocity of a sprung mass, such as the vehicle body.
2. Description of the Prior Art
Recently, there have been proposed and developed various vehicular suspension systems with variable semiactive hydraulic dampers, for actively suppressing vehicle attitude change under various vehicle running conditions and road surface conditions. One such suspension system employing variable semiactive dampers has been disclosed in Japanese Patent First Publication No. 61-163011. In this conventional system, a variable semiactive damper may be generally operated in a manner similar to the hypothetical "sky-hook" damper discussed in U.S. Pat. No. 4,742,998. A well-known control policy for such a "sky-hook" damper dictates that the damper is adjusted to a harder damping characteristic when the sign of the product of the sprung-mass absolute vertical velocity times the relative velocity between the sprung mass and the unsprung mass such as the road wheel is positive or plus, i.e., is greater than zero. Conversely, when the sign of the aforesaid product is negative or minus, i.e., is less than zero, the control policy dictates that the damper is adjusted to a softer damping characteristic. The previously noted damping characteristic control is independently performed at four variable semiactive dampers, each associated with one of the road wheels. When a semiactive damper is in its ON state, the damping coefficient thereof can be modified between a large (theoretically infinite) number of different magnitudes. In general, the damper includes a valve element having a plurality of positions to provide continuously variable damping coefficients of the damper and an actuator drivingly coupled with the valve element. Assuming that the damper is maintained at the softest damping force characteristic when the absolute vertical velocity of the sprung mass is zero, the damping control operation for the damper may be performed repetitively in the event that the absolute vertical velocity fluctuates within a slight velocity range, i.e., in the vicinity of the vertical velocity of approximately "0", due to high-frequency vibrations transmitted through the road wheel to the vehicle body during a high-speed straight-ahead driving of the vehicle on dry pavements. As set forth above, the prior art suspension system with the variable semiactive damper suffers from the drawback that the actuator is frequently energized in response to high-frequency vibrations transmitted from the road surface during the high-speed driving on dry pavements, and as a result the durability of the hydraulic damper is deteriorated and that the power consumption of the system is increased. In order to solve the above noted problem, it is advantageous to provide a predetermined dead band for the detected absolute vertical velocity of the sprung mass, so as to prevent frequent energizations of the actuator employed in the damper due to the slight velocity fluctuations. With the predetermined dead band with respect to the absolute vertical velocity of the sprung mass, since the actuator is de-energized when the value of the absolute vertical velocity is within the predetermined dead band, the frequency of energization of the actuator can be reduced. However, upon the value of the absolute vertical velocity is varied out of the dead band, the hydraulic damper may be rapidly switched to a different damping force characteristic, with the result that the damper may experience shock forces of significant magnitude. As appreciated from the above, the vehicle occupants may feel uncomfortable upon transition out of the dead band.